how language shapes the way we think.

[MaxPC]

...where do you see Sign Language fitting in (as used by the hearing impaired)? It's largely a visual language, sometimes there's auditory input but the auditory is used for emphasis. Koko the gorilla communicates with Sign and that seems to complicate this query a bit too.

I am not well informed regarding sign language which gorillas, apes, and chips have been taught, but my impression is that they use it only with humans, and do not teach it to their offspring. If this is accurate, then in that sense it is a veneer of knowledge, not something which has truly been internalized. For humans, however, language is a universal. All human cultures have spoken language. In that sense I think that the evolutionists are correct - it is an indicator of 'humanness'.

If you or others have more exact information regarding the use of sign language between animals themselves, I would find that very interesting.

I agree. There was one mention of Koko trying to teach her pet kitten to sign but like you, I think it may have been imitative behavior. Humans have the capacity to teach other and even animals the symbols and meanings of a language. Whether the student can become a teacher is another matter altogether.